I’m using RawTherapee 5.4 on Windows 7 64-bit. Whenever I do something CPU intense, task manager shows that RawTherapee peaks at 25% CPU usage. It seems like only one core of the quad-core processor (Intel Core i5-750) is used.

@ToothFairy Welcome to the forum. You have an interesting username. Unfortunately, multi-core support takes time and effort to implement in the code and also debug. I am not a dev. If you search this forum, many people have pointed this out. Take a look at how the devs responded to this question.

Most of the tools in RawTherapee are multithreaded. But there are also parts which don’t use all cores.
For example there are only a few raw decoders which use all cores (these are sony arw, float dng, phase one iiq, Fuji compressed). Canon cr2 decoder uses 2 cores. Most of the other decoders use only 1 core.

After decoding the raw files, almost all tools use all cores, with the exception of Tonemapping tool and maybe one or two other tools.

I’ve just tried some things like opening photos, switching between them and saving them and now I did see the CPU usage go up way beyond 25%. I’m not sure what I did previously that CPU always peaked at 25% because now I can’t seem to reproduce it.